Bleaching of chemical pulp for papermaking is attained in multistage bleaching treatment. Heretofore, in the multistage bleaching, a chlorine-base bleaching chemical is used as the bleaching agent. Concretely, bleaching is attained by a combination of chlorine, hypochlorite and chlorine dioxide, for example, in a sequence of C-E-H-D or C/D-E-H-E-D.
In the above, “C” means a chlorine treatment stage; “H” means a hypochlorite treatment stage; “D” means a chlorine dioxide treatment stage; “E” means an alkali treatment stage. “-” means that, after the step of the treatment stage described just before “-”, the system is washed, and then, the step of the treatment stage described just after “-” is carried out. “/” means the treatment stage where the chemicals described just before and just after “/” are used in combination; and for example, “C/D” means a treatment stage where chlorine and chlorine dioxide are used in combination.
However, in bleaching, these chlorine-base bleaching chemicals release, as side products, organic chlorine compounds that are harmful to the environment, and the environmental pollution with the bleaching waste that contains those organic chlorine compounds is considered problematic. Organic chlorine compounds are analyzed and assessed generally by an AOX method, for example, by the US Environment Agency (EPA: METHOD-9020).
For reducing and preventing release of organic chlorine compounds as side products, it is most effective to reduce the amount of chlorine-base chemicals to be used or not to use them; and in particular, it is the most effective process not to use molecular chlorine in the initial stage. Pulp produced according to the process is referred to as ECF (elementary chlorine-free) pulp; and pulp produced according to a process using no chlorine-base chemical at all is referred to as TCF (totally chlorine-free) pulp.
As a process for bleaching cooked and oxygen-deligninated pulp by no use of molecular chlorine in the initial stage, generally known is bleaching by use of a chlorine dioxide treatment stage as the initial stage according to a sequence of D-Eo-D, D-Eop-D, D-Eo-D-D, D-Eop-D-D, D-Eo-P-D or D-Eop-P-D, or bleaching by use of an ozone treatment stage as the initial stage according to a sequence of Z-Eop-D, Z-Eo-P-D or ZD-Eop-D.
In the above, “Z” means an ozone treatment stage; “P” means a hydrogen peroxide treatment stage. “p” means hydrogen peroxide; “o” means oxygen. “Eo” means an alkali treatment stage combined with oxygen; “Eop” means an alkali treatment stage combined with oxygen and hydrogen peroxide. “ZD” means continuous treatment of an ozone treatment stage (Z) and a chlorine dioxide treatment stage (D) with no washing between the two stages. The others are the same as mentioned above.
However, chlorine dioxide and ozone are inferior to conventionally used chlorine in point of the ability to remove hexeneuronic acid (which may be referred to as “HexA”), and therefore a large amount of HexA remains in the bleached pulp. The remaining HexA is a cause of the deterioration of the colour reversion resistance of ECF- or TCF-bleached pulp.
Hexeneuronic acid (HexA) is a substance resulting from demethanolation in a cooking step of α-glucuronic acid bonding to xylan, which is a hemicellulose existing in pulp. Though HexA has a small influence on the whiteness of pulp, it reacts with potassium permanganate, because of having a double bond in the molecule, and is counted as a K value or a kappa value; and this consumes a bleaching agent such as chlorine dioxide, ozone, etc.
As a papermaking process, there are known an acid papermaking process of using aluminium sulfate, and a neutral papermaking process of using calcium carbonate. Neutral paper worsens in colour reversion resistance with the increase in the HexA content thereof, but the degree of worsening is low; and it is the acid paper made by use of aluminium sulfate that particularly worsens in colour reversion resistance. The reason why the acid paper made according to an acid papermaking process worsens in colour reversion resistance is unknown at present, but the existence of HexA and the use of aluminium sulfate may be the reason for it.
In a paper mill, in general, neutral paper and acid paper are made separately from chlorine-free bleached pulp forwarded from a series of bleaching equipment, by use of a large number of papermaking machines. Accordingly, using the same chlorine-free bleached pulp forwarded from the same bleaching step, acid paper is made in one line and neutral paper is made in another line. In this case, even though the neutral paper made in the neutral papermaking line has no problem of the colour reversion resistance, the acid paper made in the acid papermaking line may have a problem of the colour reversion resistance.
As a method for improving this worsening of the colour reversion resistance, there is a method of removing HexA by increasing the amount to be used of chlorine dioxide or ozone having the ability to remove HexA. In this case, however, the pulp for neutral paper not requiring measures against the colour reversion must also be bleached, and it causes problems of too much increase in whiteness of the neutral paper and great increase in the cost for bleaching.
For solving the problems, proposed is a process of applying monopersulfuric acid (which may be referred to as “MPS”) to bleaching, as mentioned below.
In place of chlorine bleaching treatment or delignination treatment by a combination of chlorine and chlorine dioxide, proposed is a TCF bleaching process of bleaching unbleached pulp by monopersulfuric acid treatment followed by alkaline hydrogen peroxide treatment (see Patent Reference 1). This process relates to initial stage delignination in the bleaching step; however, the reference describes nothing relating to the process of the present invention of treatment with monopersulfuric acid used in combination in the chlorine dioxide treatment stage, and to HexA removal, improvement of colour reversion resistance and viscosity reduction inhibitory effect.
In place of chlorine bleaching treatment or delignination treatment by a combination of chlorine and chlorine dioxide, proposed is a process of bleaching unbleached pulp by treatment with a combination of enzyme and monopersulfuric acid (see Patent Reference 2). This process relates to initial stage delignination in the bleaching step; however, the reference describes nothing relating to the process of the present invention of treatment with monopersulfuric acid used in combination in the chlorine dioxide treatment stage, and to HexA removal, improvement of colour reversion resistance and viscosity reduction inhibitory effect.
In place of chlorine bleaching treatment or delignination treatment by a combination of chlorine and chlorine dioxide, proposed is a process of bleaching unbleached pulp by treatment with a chelating agent, treatment with an alkaline hydrogen peroxide and treatment with monopersulfuric acid after oxygen bleaching (see Patent Reference 3). This process relates to initial stage delignination in the bleaching step; however, the reference describes nothing relating to the process of the present invention of treatment with monopersulfuric acid used in combination in the chlorine dioxide treatment stage, and to HexA removal, improvement of colour reversion resistance and viscosity reduction inhibitory effect.
In place of chlorine bleaching treatment or delignination treatment by a combination of chlorine and chlorine dioxide, proposed is a process of bleaching unbleached pulp by treatment with a combination of monopersulfuric acid and ozone (see Patent Reference 4). This process relates to initial stage delignination in the bleaching step; however, the reference describes nothing relating to the process of the present invention of treatment with monopersulfuric acid used in combination in the chlorine dioxide treatment stage, and to HexA removal, improvement of colour reversion resistance and viscosity reduction inhibitory effect.
For delignination process, proposed is a process of bleaching unbleached pulp by monopersulfuric acid treatment followed by alkaline hydrogen peroxide treatment after chelating agent treatment (see Patent Reference 5). This process relates to initial stage delignination in the bleaching step; however, the reference describes nothing relating to the process of the present invention of treatment with monopersulfuric acid used in combination in the chlorine dioxide treatment stage, and to HexA removal, improvement of colour reversion resistance and viscosity reduction inhibitory effect.
As a process of bleaching unbleached pulp, proposed is a process of treatment with a peracid and an alkaline earth metal in the final stage of bleaching (see Patent Reference 6). Monopersulfuric acid for use in the present invention is also a peracid; however, the process proposed in the reference differs at all from the process of the present invention in that, in the present invention, treatment with monopersulfuric acid used in combination in the chlorine dioxide treatment stage is performed and an alkaline earth metal is not used along with monopersulfuric acid. Further, in Patent Reference 6, peracetic acid is used as the peracid; however, the main object of the process is for increasing the whiteness, and the reference describes nothing relating to HexA removal, improvement of colour reversion resistance and viscosity reduction inhibitory effect.
As a process of post treatment after bleaching, proposed is a process of adding a bleaching agent between the bleaching step and a preparation step (see Patent Reference 7). Patent Reference 7 describes, as the bleaching agent, ozone, hydrogen peroxide, peracetic acid, percarbonic acid, perboric acid, and thiourea dioxide; however, the main object of this process is for increasing the whiteness, and the reference has no description relating to HexA removal and improvement of colour reversion resistance.
As a process of post treatment after bleaching, the present inventors have proposed a process including monopersulfuric acid treatment between the bleaching step and the preparation step (see Patent Reference 8). In this process, the finished pulp is treated with monopersulfuric acid to remove HexA from it and to improve the colour reversion resistance thereof; however, this suggests nothing relating to the process of the present invention that uses monopersulfuric acid in the chlorine dioxide treatment stage and to the effect of inhibiting the pulp viscosity reduction.
As a process of using monopersulfuric acid treatment in the initial stage of bleaching, the present inventors have proposed a process including treatment with an inorganic peroxide followed by multistage bleaching treatment starting from treatment with chlorine dioxide (see Patent Reference 9). In this process, monopersulfuric acid treatment is introduced into the initial stage of bleaching for removal of HexA and for improvement of colour reversion resistance. However, this process has some problems in that the pulp viscosity reduction is great owing to introduction of the monopersulfuric acid treatment thereinto and that the process is hardly usable in a papermaking process that requires high paper strength. Another problem is that the process requires an additional monopersulfuric acid treatment tower at the top of the conventional bleaching equipment for which the investment cost for the additional equipment is great.
Patent Reference 9 suggests nothing relating to the process of the present invention where monopersulfuric acid is used in combination in the chlorine dioxide treatment stage.    [Patent Reference 1] JP-T 6-505063    [Patent Reference 2] JP-A 7-150493    [Patent Reference 3] JP-T 8-507332    [Patent Reference 4] JP-T 8-511308    [Patent Reference 5] JP-T 10-500178    [Patent Reference 6] JP-T 2001-527168    [Patent Reference 7] JP-A 2004-169194    [Patent Reference 8] JP-A 2007-169831    [Patent Reference 9] WO 2007/132836